Have you noticed that code reuse works these days? For a long time, software engineers struggled with the difficulty of reusing existing software, but it's now common place
Let me give you some examples. I use Linux, Nginx, MySQL, and Python, not to mention a Web browser. These days, very few people need to write a custom kernel, Web server, database, or programming language to solve their particular problem. Sure it happens, but it's far more common to reuse something existing.
I even make use of an existing Web framework, Pylons, and an existing templating engine, Mako. Those things are often written from scratch, but I didn't need to. They were fine.
Even within my own code, I find plenty of places for reuse. Each of my clients has a pretty different setup. Their input formats and output formats are often pretty different, but by using a UNIXy "small tools that can be pieced together" approach, I usually write only a small amount of code when I get a new customer.
What has changed? Why is it suddenly so easy to reuse code? Has object-oriented programming finally paid off? Maybe. However, I think the more likely culprit is open source. Small companies are now viable because they have access to a huge corpus of freely available source code. They don't have to pay for it. They can look at the source if the documentation is inadequate. They can contribute bug fixes if they encounter bugs. They can even hack it in deep ways to accomplish special tasks. This is particularly common in the BSD world.
Last of all, testing and a strong dedication to docstrings help me with reusing my own code. Per agile thinking, I don't try to get it right the first time. If I need to add a feature to make use of code in an unexpected way, I can. The docstrings help me understand what's already there, and the tests help make sure I don't break it.